This invention relates to a switched input circuit structure for operation at low supply voltage levels. Specifically, the invention relates to a switched input circuit structure of the type having an input terminal on which an input voltage is received and an output terminal connected to an input capacitor.
1. Background of the Invention
The invention concerns in particular, but not exclusively, a switched input circuit structure of the serial type in SC configurations, and reference will be made in the ensuing description to this field of application for convenience of illustration.
2. Background Art
As is well known, when operating at low supply voltage levels, e.g., below 1.2V, it becomes difficult to ensure that switches incorporated to circuits of the SC type are fully closed. Especially critical in this respect are serial switches, that is, switches which are connected in series with the flow of the main signal in the circuit which contains them. These serial switches are required to be in the xe2x80x9conxe2x80x9d state at each value of the main signal swing.
This problem has been solved for circuits which include operational amplifiers by means of arrangements known as switched-opamps. The critical switches placed at the output of each operational amplifier are replaced by a step of turning on/off the operational amplifier. This solution obviously cannot be applied to input signals, whose source is normally taken to be a very low impedance one.
FIG. 1 shows schematically an input circuit structure 1 of an SC circuit including, in particular, an input terminal I1 and a series of an input switch S1 and an input capacitor Cin. This input circuit structure 1 further includes a controlled power-on switch S2 connected between a central connection node X, intervening between the input switch S1 and the input capacitor Cin, and a voltage reference for instance the supply VDD.
While the power-on switch S2 can be optimized by suitably selecting the value of its reference voltage V2, the input switch S1 cannot be closed when the input voltage Vin received by the input terminal I1 approaches a value halfway toward the supply voltage VDD, namely a value VDD/2, as shown schematically in FIG. 2.
More particularly, FIG. 2 is a plot of the typical conductance gds for an input switch as the input voltage Vin applied thereto varies. A comparison of the graphs shown in FIG. 2 brings out the critical aspects of the input switch operation at low values of the supply voltage VDD, e.g., of 1.2V.
In particular, it is possible to appreciate that for supply voltage values Vdd of at least 5 V, the transconductance value gds assumes a value higher than zero around Vdd/2 while it assumes the zero value for a supply voltage value Vdd=1.2 V. No other types of input switches are currently known which can operate in a condition of low supply voltage.
The underlying technical problem of this invention is to provide a serial switch for circuits of the SC type, which has such structural and functional features that it can operate correctly even at low values of the supply voltage.
Accordingly, the present invention provides a switched input circuit structure of the type which includes an input terminal receiving an input voltage, and an output terminal connected to an input capacitor. The structure comprises an operational amplifier having a non-inverting terminal connected to a ground reference terminal, an inverting input terminal, and an output terminal feedback connected to the inverting input terminal and held in a virtual ground condition by a parallel of first and second charge paths which are connected between the input terminal of the switched input circuit structure and the inverting input terminal of the operational amplifier and connected to the supply voltage reference and the ground reference, respectively.
Additional objects, advantages, novel features of the present invention will become apparent to those skilled in the art from this disclosure, including the following detailed description, as well as by practice of the invention. While the invention is described below with reference to preferred embodiment(s), it should be understood that the invention is not limited thereto. Those of ordinary skill in the art having access to the teachings herein will recognize additional implementations, modifications, and embodiments, as well as other fields of use, which are within the scope of the invention as disclosed and claimed herein and with respect to which the invention could be of significant utility.